Gold based alloy composition and brazing therewith, particularly for ceramic-metal seals in electrical feedthroughs

ABSTRACT

New alloys containing gold, vanadium, yttrium and/or scandium, optionally including niobium. The alloys are particularly suitable for brazing and for metallizing, most particularly for brazing hermetic ceramic-metal seals. Unique brazing structures and methods are also disclosed.

This is a division, of application Ser. No. 885,489, filed Mar. 13,1978.

BACKGROUND OF THE INVENTION

This invention relates to new alloys containing gold, vanadium, and, asa third constituent, yttrium or scandium or mixtures thereof.Additionally, the alloys may optionally contain niobium. In compositionsincluding minor amounts of vanadium, yttrium and/or scandium, optionalminor amounts of niobium, and a major amount of gold, the alloys areparticularly useful as brazing materials and find a preferred use in theformation of hermetic corrosion-resistant seals or brazed joints forelectrical lead-ins, particularly of the ceramic-metal seal types. Themost preferred use of these alloys lies in the manufacture ofceramic-metal electrical lead-ins for implantable electromedical devicessuch as cardiac pacemakers and the like.

The invention also relates to the use of high gold content alloys forthe metallization of ceramic materials such as alumina and otherceramics.

An important feature of the invention lies in the direct application ofthe brazing alloy to a ceramic-metal seal without prior metallization ofthe ceramic. For example, if the ceramic is alumina, it may be brazedwith these alloys without prior metallization.

SUMMARY OF THE INVENTION

Broadly, this invention provides novel alloys of gold, vanadium, yttriumand/or scandium, optionally including niobium.

It has also been found, in accordance with this invention, that alloycompositions comprising a major portion of gold coupled with minorportions of vanadium and/or yttrium or scandium and optionally includingniobium, provide excellent brazing and metallizing materials.

The alloys within the scope of this invention contain vanadium as anessential constituent. In the preferred compositions, the vanadiumranges from about 4 atomic percent up to about 15 atomic percent.Another essential constituent is selected from the group consisting ofyttrium, scandium, and mixtures thereof. In the preferred alloycompositions, the yttrium and/or scandium ranges from about 0.008 atomicpercent up to about 0.2 atomic percent. The optional constituent niobiumis preferably present in minor amounts ranging from about 0 to about 3atomic percent. The balance of the composition is gold, preferably inmajor amount. The percentages, as noted above, are expressed on anatomic percentage basis which is used throughout, unless otherwisenoted.

In the most preferred alloy compositions of the invention, the amount ofvanadium ranges from about 4% up to about 5% and the selectedconstituent yttrium and/or scandium ranges from about 0.008% up to about0.02% in amount. If niobium is added, as optionally provided, it willpreferably range between about 0% to about 3% in amount.

The most preferred alloy of the invention consists essentially of about5% vanadium, about 0.02% yttrium, balance gold. Optionally, it maycontain about 0.5% niobium.

The invention not only contemplates within its scope novel alloycompositions for various uses, but also novel brazed structures. Thestructural novelty of the brazed structures may arise either from thenovel brazing alloy compositions used or from unique structural featuresdescribed hereinbelow.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 and 2 are cross sections of an electrical lead-in in the form ofa ceramic-metal seal using an alloy of the invention. FIG. 1 shows thestructure before it is brazed. FIG. 2 shows the structure after it isbrazed and mounted in an electrical device.

FIGS. 3 and 4 show, before and after brazing respectively, a uniquestructural feature for aiding the "wetting" of a brazing material in aceramic-metal seal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As has already been pointed out, this invention relates in general tothe use of high gold content alloys for the brazing and metallization ofvarious ceramic materials, such as alumina (Al₂ O₃), and various metals.Specifically, in its preferred form, it relates to the use of thesealloys in the manufacture of ceramic-metal electrical lead-ins,particularly for implantable electromedical devices such as cardiacpacemakers.

The alloys of the invention, as characterized by the nature of theirconstituents, consist essentially of a carrier constituent, an activeconstituent, and a catalytic constituent. The carrier constituentcomprises an element which imparts to the alloy the desired corrosionresistance and mechanical properties. In addition, in the case ofimplantable electromedical devices such as cardiac pacemakers, thecarrier constituent must also provide biocompatability. The carrierconstituent is the major component of the alloy. In the alloys of thisinvention, the carrier constituent is gold (Au).

The active constituent of the alloys of this invention consists of anelement or elements which may chemically react with the material to bemetallized or with one of the materials to be brazed. For example, in apreferred form of this invention, an alumina ceramic material is brazedto a metal and the active constituent of the alloy chemically reactswith the alumina and is thus responsible for the bonding of the ceramicto the metal.

The active constituent in accordance with the invention is selected fromthe class of metals commonly identified as strong oxide formers. Manymetals exhibit a strong thermodynamic tendency to form oxides. However,the element selected for the alloys of this invention must also form asolid solution, i.e., a single phase structure with the carrierconstituent. This requirement significantly limits the active elementswhich are suitable for a gold carrier constituent. The single phasemicrostructure ensures the retention of the desirable propertiescharacterizing the carrier constituent of the alloy. Exceeding the solidsolution solubility limit results in the formation of intermetalliccompounds which generally impair the corrosion resistance and ductilityof the alloys.

Of the various elements meeting the above requirements, it has beendetermined that only one element, i.e., vanadium (V), is satisfactoryfor the purposes of this invention. Vanadium has a solubility limit ofapproximately 15% in gold, which establishes the upper limit for thisconstituent in alloys according to this invention.

Other elements such as tantalum and hafnium, which exhibit very slightsolubilities in gold as discussed above, are not acceptable except inincidental amounts since it has been determined that these elements donot significantly enhance wetting of the alloy when included in thealloys of the invention in any significant amount.

Various impurities may be present in the alloy in incidental amounts. Itis preferred that they not exceed a total of about 0.1 wt. % of thetotal composition.

Another element, i.e., niobium (Nb), has been found to be an acceptableconstituent when included in the alloys of the invention if its atomicpercentage is maintained below about 3%.

However, the niobium is not a substitution for the vanadium, butcomprises an optional addition to the overall composition. It does notaffect wettability of the alloy in amounts below about 3% and it appearsto improve corrosion resistance.

Another essential constituent of the alloys of this invention is thecatalytic constituent which enhances the rate of the chemical reactionbetween the ceramic portion of a seal or lead-in structure and theactive constituent of the alloy, i.e., the vanadium.

The most preferred catalytic constituent is yttrium (Y) which has beenfound to greatly accelerate the chemical reaction between a ceramic suchas alumina and the vanadium and/or vanadium-niobium. The increasedreaction rate may be due to the ability of yttrium to deoxidizevanadium, thus maintaining it in a highly chemically active state, orthe yttrium may simply catalyze the reaction of the vanadium and theceramic. An additional benefit realized from the yttrium addition isthat it prevents the brazing alloy from "balling up" on the ceramicsurface. Hence, the molten alloy will wet that portion of the ceramicwith which it is in physical contact when in the solid state but willnot flow on the surface thereof.

Scandium (Sc) has also been found to catalyze the reaction of thevanadium with ceramics. Scandium may be used to replace the yttrium orthe two elements may be used mixed together in any proportion.

In assessing the upper and lower limits of the optimum compositionalranges for the various constituents in accordance with the invention,alloy wafers of about 1/8" in diameter were prepared. The wafers wereplaced upon sapphire sheets and heated to their melting point. Theresultant samples were evaluated as to shape retention after beingheated in excess of their melting point. Any substantial loss of shapewas taken as being indicative of poor wetting and loss of bond area;both of which are unacceptable, particularly when a hermetic seal isdesired.

Based on such tests, it was determined that when the amount of vanadiumdecreases below about 4%, a noticeable loss of "wetting" begins todevelop. When the vanadium is increased above about 10-15%, nonoticeable increase in "wetting" appears to occur, and the alloy suffersa noticeable loss of ductility and corrosion resistance. Theseproperties appear to be optimal within the narrower range of from about4% to about 5% vanadium.

With regard to the yttrium and/or scandium constituent, amounts belowabout 0.008% are difficult to measure reproducibly, and amounts greaterthan or equal to 0.2% tend to foster the formation of intermetalliccompounds which decrease the corrosion resistance of the alloys andincrease their brittleness. The optimum range was determined to be fromabout 0.008% to about 0.02%.

Amounts of niobium up to about 3% do not affect ductility orwettability. Furthermore, 0.5% Nb in Au-5V-0.02Y did not causebrittleness and is believed to improve the corrosion resistance of thealloy. The optimum amount was determined to be about 0.5% niobium.

It was established then that the alloys preferably contain: from about4% up to about 15% vanadium; from about 0.008% up to about 0.2% ofyttrium; the balance being gold.

However, scandium can be substituted in whole or in part for theyttrium, in the alloys.

The most preferred compositions will contain: about 5% vanadium; about0.02% yttrium or equivalent (i.e., Sc substituted in whole or in partfor Y); balance gold.

Additionally, in any of the compositions, niobium in an amount of from0% to about 3%, 0.5% being preferred, can additionally be added to theother constituents.

Of the various alloys contemplated herein, the gold-vanadium-yttriumternary alloy is the most preferred. The optimum composition for thisternary alloy is about 5% vanadium, about 0.02% yttrium, balance gold.

EXAMPLE 1

An alloy of Au-5% V-0.02% Y was placed on a sapphire substrate andmelted. The contact angle for this alloy was less than 90°. It wet thesapphire rapidly and did not "ball-up" after melting.

EXAMPLE 2

An alloy of Au-5% V-0.02% Y was observed to have a microstructure whichshowed no evidence of intermetallic compound function when viewed at1000 X.

This alloy was flattened into a disc of approximately 0.015 inches thickand cut into a "half moon" shape. It was placed on a sapphire substrateand melted in vacuo. The alloy was observed to wet the substrate andretain its "half moon" shape.

EXAMPLE 3

Au-4.99% V-0.21% Y

Same treatment and results as for Example 2.

EXAMPLE 4

Au-5% V-0.008 Y

Same treatment and substantially the same results as for Example 2.

EXAMPLE 5

Au-4% V-0.02 Y

Same treatment and substantially the same results as for Example 2.

EXAMPLE 6

Au-5% V-0.02% Y

Same treatment and substantially the same results as for Example 2.

EXAMPLE 7

Au-5% V-0.02% Sc alloy formed into a disc of about 0.250 inches diameterand placed on a sapphire substrate.

Upon heating in vacuo to melting, the disc wet the sapphire and retainedits original shape.

EXAMPLE 8

Au-5 V-0.01% (Y+Sc) alloy

Same treatment and results as Example 8.

EXAMPLE 9

Au-5% V-0.5% Nb-0.02% Y

Alloy--Same treatment and results as Example 8.

The alloys of the invention possess several advantages for use as abrazing material in forming ceramic-metal seals.

The alloys comprise a combination brazemetallization alloy for ceramicssuch as alumina. Metallization and brazing are accomplished in a singlestep during brazing.

The alloys exhibit excellent corrosion resistance and biocompatability.The alloys do not exhibit excessive flow properties.

The alloys exhibit a high degree of plasticity (fracture strain of about20%) as well as low yield strength (about 12,000 psi) and ultimatetensile strength (about 30,000 psi) by virtue of the solid solutionmicrostructure and the small degree of alloying needed to achievebonding between the metal and ceramic in a seal or lead-in structure.These mechanical properties also tend to minimize thermal stresses whichcan induce cracking in the ceramic. Furthermore, these mechanicalproperties allow the alloy to be easily prepared by conventionalmethods.

The alloys flow readily but only by capillary action and readilymetallize recesses in the joint between the ceramic and metal.

The interfacial bond between metal and ceramic is high quality, andthere is no tendency for the braze to fracture the ceramic.

The alloy bonds to a wide variety of ceramics in addition to alumina(single crystal sapphire as well as polycrystalline alumina) such ascarbon, forsterite, steatite, zircon, and mullite. Other refractorymetal oxides such as zirconia, thoria, yttria, silica, and magnesia mayalso be considered for use.

In preparing the alloys of the invention, the gold and otherconstituents are charged onto a water-cooled copper hearth or chillplate, in weighed increments, in an arc melting furnace and melted ineither an inert atmosphere such as argon or helium or in a vacuum toprevent contamination of the melt and loss of the constituents due tooxidation. The constituents may be melted several times for uniformityof the overall composition. The resulting melt is then solidified andprepared for use in any form desired by conventional working techniques.It may, for example, be rolled to foil thickness and cut to any desiredshape.

In whatever form prepared, the alloy, when used as a brazing alloy, ispositioned about the surfaces to be joined and placed in a furnace, thejoint and surrounding brazing material are heated, in vacuo or under aninert atmosphere such as argon or helium, and held at temperature untilthe melted alloy is observed to melt and flow freely between thesurfaces to be joined. At this point, the heating means is turned offand the melted alloy allowed to solidify to form the brazed joint.

The brazing alloys of this invention generally have a melting point ofabout 1960° F., and, when brazing, the temperature is usually raised toabout 50° F. above the melting point of the alloy. Brazing at about2000° F. to about 2025° F. for about two minutes from initial meltinghas been found to be satisfactory in brazing most ceramic-metal seals.The brazing operation must be conducted in a protective atmosphere,i.e., in vacuum (about 10⁻⁴ torr or less preferred) or under an inertatmosphere (dew point of about 90° F. or less preferred) such as heliumor argon to reduce any contamination in the brazed joint or oxidation ofthe alloy constituents.

The parts to be brazed with the alloy should be cleaned and prepared byconventional physical or chemical methods before brazing. It is notnecessary to metallize before brazing as excellent joints are preparedwithout this procedure.

As a brazing alloy, the alloys of this invention are capable of beingused in a foil form or any preform shape. For example, a preform washershape is preferred for electrical lead-ins of the coaxial type as aredescribed hereinbelow.

Referring now to FIGS. 1 and 2, an electrical lead-in incorporating aceramic-metal seal prepared with the most preferred alloy of theinvention is shown before and after brazing. The lead-in is orientedwith the interior end thereof positioned upwardly in the figures. Thelead-in comprises a central electrical conductor 10, which may take theform of a pin surrounded by a sapphire or alumina insulating body 12.Body 12 is encased in a metal ferrule 14 which, when the lead-in isinstalled in a device, may be welded or soldered into a wall 16 thereofas illustrated in FIG. 2. Pin 10 may be any conductive metal. However,in the case of an implantable medical device, it is preferred that thepin comprise niobium, tantalum, platinum, palladium, or titanium,niobium being preferred. Likewise, for such an application, alumina orsapphire is preferred for the ceramic, although, as pointed out hereinabove, other insulating ceramic-like materials may be used. The metalferrule for implantable use is preferably titanium.

As can be seen in FIG. 1, to accomplish brazing, one or more preformwashers 18 comprising an alloy of the invention, (V-5.5%, Y-0.2%, balAu, for example) may be placed on ceramic body 12, surrounding pin 10,and on metal ferrule 14 surrounding ceramic body 12. The size of washer18 and/or the number of washers used may vary as the flow of the liquidbraze depends on the particular geometric characteristics of the articleto be brazed.

Heating the assembly with the washer preforms positioned, as shown inFIG. 1, to a temperature of approximately 2000°-2025° F. for about twominutes in a protective atmosphere will, upon cooling, provide thebrazed structure as shown in FIG. 2 having two brazed joints 22. Thelead-in may then be welded or soldered into an opening in a device wall16 as illustrated in FIG. 2.

FIGS. 3 and 4 are illustrative of a preferred modification which may beused in electrical lead-in structures for facilitating the wetting ofthe brazing material to the conductive pin 10 and also for facilitatingthe flow of the melted brazed material into a joint area. Only thecentral pin 10 and the ceramic insulator 12 are shown in the figures.Pin 10 is provided with a peripherally extending shoulder means 24 whichoverlaps ceramic insulator 12 and contacts the brazing material preform18. Preform 18 may be in the form of a washer as above described.Heating of the assembly to brazing temperature and cooling results in abrazed structure similar to that shown in FIG. 4 in which the flow ofthe brazing material into the joint area is facilitated by the contactbetween shoulder 24 and preform 18. Shoulder 24 may be simply formed by"upsetting" the pin using standard metal working techniques.

Corrosion resistance of the alloys of this invention was tested in twoways. Lead-ins brazed with representative alloy samples were subjectedto life testing in pacemakers, and anodic polarization tests inaccordance with ASTM G5-71 were conducted on other representative alloysamples.

Having described the invention by reference to preferred embodiments byway of illustration, exclusive property rights therein are defined bythe following claims.

What is claimed is:
 1. An alloy particularly suitable for brazing andmetallizing consisting, in addition to minor amounts of impurities andincidental elements if any, essentially of gold in a predominant amount,a minor amount of vanadium, a minor amount of a constituent selectedfrom the group consisting of yttrium, scandium and mixtures thereof, andoptionally including a minor amount of niobium.
 2. An alloy according toclaim 1 which consists on an atomic percentage basis essentially of:from about 4% up to about 15% vanadium; from about 0.008% up to about0.2% of the constituent selected from the group consisting of yttrium,scandium, and mixtures thereof; from 0% up to about 3% niobium, thebalance of the alloy being gold and impurities and incidental elements,if any.
 3. An alloy according to claim 2 wherein the amount of thevanadium is from about 4% to about 5%.
 4. An alloy according to claim 1wherein the selected constituent is yttrium in an amount of about 0.02%.5. An alloy according to claim 2 including niobium in an amount of about0.5%.
 6. The alloy according to claim 2 wherein the amount of thevanadium ranges from about 4% up to about 5% and the selectedconstituent ranges from about 0.008% up to about 0.02% in amount.
 7. Analloy according to claim 6 including niobium in an amount of about 0.5%.8. An alloy according to claim 2 wherein the amount of the vanadium isabout 4-5% and the selected constituent is yttrium in an amount of about0.02%.
 9. An alloy according to claim 8 including niobium in an amountof about 0.5%.
 10. A brazing alloy having a melting point in excess ofabout 1900° F. and capable of forming hermetic and corrosion resistantjoints, the alloy consisting, in addition to impurities and incidentalelements, essentially of gold in a major amount and the followingconstituents in minor amounts: vanadium, a constituent selected from thegroup consisting of yttrium, scandium, and mixtures thereof, andoptionally including niobium.